The present invention relates generally to methods and systems for radiocommunications and, more particularly, to such systems in which communication quality is monitored and connections can be handed over from one channel or base station to another based on monitored quality.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry""s growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
In cellular systems, the capability is typically provided to transfer handling of a connection between, for example, a mobile station and a serving base station to another, neighboring base station, as the mobile station changes its position and so moves out of the coverage area of one base station and into the coverage area of another base station. This type of handoff is commonly referred to as an xe2x80x9cintercellxe2x80x9d handoff as the coverage areas associated with base stations are commonly referred to as xe2x80x9ccellsxe2x80x9d. Depending upon the quality of the current channel, it may also be desirable to transfer a connection from one channel of the base station to another channel supported by the same base station, which handoffs are commonly referred to as xe2x80x9cintracellxe2x80x9d handoffs.
To smoothly complete a handoff, the network controlling the base stations first determines, for each mobile station, whether the need for handoff from its currently serving base station is imminent and secondly determines to which new base station (or channel) handoff should be effected. In making the former decision, the radiocommunication system monitors the quality of the mobile station""s current connection. In making the latter decision it is desirable that the network controller know either how well each neighboring base station can receive signals from a mobile station in question, how well the mobile station in question can receive signals from each neighboring base station, or both. Monitoring signalling associated with a serving base station and neighboring base stations typically involves making signal strength measurements, which measurements can be made in a variety of ways depending, for example, on the type of access methodology employed by the radiocommunication system.
Many existing radiocommunication systems are based on an access method known as Frequency Division Multiple Access (FDMA), in which each mobile station transmits on a unique frequency within its current base station area. The mobile station is thus unaware of signals on other frequencies from surrounding base stations. In FDMA systems it is typically considered too costly to equip mobile stations with an extra receiver that could be used to scan other base station frequencies to determine their received signal strength. Instead, it is common for base stations to be equipped with a scanning receiver that searches for the signals of approaching mobile stations. When the base stations scan for the signal strength of mobile stations, the method could be termed base assisted handover (BAHO). The network then hands over a mobile station from a base station covering an area that the mobile is leaving to the base station that reports the best reception of the mobile station""s signal.
More recent cellular telephone standards employ Time Division Multiple Access (TDMA) in which a fixed time period (e.g., 20 mS) on each pair of radio frequency links (one used in the direction from mobile station to base station (uplink), the other used in the direction from base station to mobile station (downlink)) is divided into a number (e.g., 3) of short timeslots (e.g., 6.6 mS) that are cyclically used by different mobile stations. Thus, a first mobile station transmits in the first timeslot in each period, a second mobile station transmits in the second timeslot in each period and so on. Likewise the base station transmits to one mobile station in the first timeslot, another mobile station in the second slot and so on. By offsetting the allocation of timeslots in the two communication directions, base to mobile (the downlink) and mobile to base (the uplink), it can be arranged that a first mobile transmits in the first timeslot and receives in the second timeslot; a second mobile transmits in the second timeslot and receives in the third, while a third mobile transmits in the third timeslot and receives in the first timeslot. An advantage of this arrangement is that a mobile station does not need to transmit and receive simultaneously, which reduces disturbance.
In the above three-timeslot example, each mobile station is active to transmit or receive in two of the three timeslots and is idle in the remaining timeslot. Therefore it is possible for TDMA mobile stations to use this idle time to receive signals from other base stations and measure their signal strength. Typically, such measurements are made on the control channels (or some type of other measurement channel) being broadcast by these base stations, rather than the traffic channels used to support circuit-switched or packet-switched connections.
By reporting these signal strength measurements to the base station using a slow speed data channel multiplexed with the traffic (e.g., voice), the network is informed about the base stations each mobile station can receive. The network can use this information to effect handoff to the preferred base station, and such a method is termed mobile assisted handover (MAHO). For example, the system can compare the measured signal strengths associated with neighboring base stations to the signal strength at which the mobile station is receiving transmissions from its serving base station. If a neighboring base station is received at a sufficiently higher signal strength (e.g., employing a hysteresis value to avoid a xe2x80x9cping-pongxe2x80x9d handoff effect at cell borders), than the serving base station, then a handoff is performed. Systems providing MAHO can also have access to the base station measurements, and so are able to effect smoother and more reliable handovers because both uplink and downlink signal strengths are taken into account, instead of just uplink strengths in the case of BAHO.
In yet another access technique, Code Division Multiple Access (CDMA), mobile stations can share the same frequency band but communications are distinguishable by virtue of unique spreading codes. Even in CDMA systems it is possible to measure a signal strength of pilot channels associated with a particular base station. The base station and/or mobile station can use this information to determine when a handover to another code, or another frequency band in multicarrier CDMA, is desirable.
As can be seen from the foregoing, a common element in determining when a handoff is desirable involves the measurement and comparison of received signal strengths, and in particular measurements made on control channels of various neighboring base stations. However, there are several complicating factors relating to the output transmit powers of the transceivers within base stations which reduce the accuracy of conventional handoff algorithms.
The actual output transmit power of a particular transceiver can vary from transceiver to transceiver within a base station for a number of different reasons. For example, problems or changes to components within a transceiver may cause its actual output transmit power to vary from an intended value. Other factors which may cause variances in actual output transmit power between transceivers involve different power settings which are intended to permit, for example, a network operator to have greater control over system operation.
For example, control channels and traffic channels are not always transmitted by base stations using the same transmit power. In some systems it is possible to transmit control channels at a higher power level than traffic channels in the same cell to ensure that the control information contained in the control channels will be accurately received by the potentially large number of mobile stations that are listening thereto. To enable, for example, a network operator, to control a transmit power difference between control channels and traffic channels a power setting for traffic channels (PSVB) and a power setting for measurement channels (PSMB) can be provided. These two settings can also be combined into one differential power setting, e.g., PSVB-PSMB. The PSVB and PSMB power settings (or differential power setting) are stored in the mobile switching centers (MSCs) of radiocommunication systems for each base station, which base station typically includes a plurality of transceiver devices.
In addition to the PSVB/PSMB power settings, network operators can also set the basic output power of each transceiver device individually. Like the PSVB/PSMB power settings, these individual transceiver power settings can also be stored in the MSC.
From the foregoing, it will be apparent to those skilled in the art that the actual output transmit power of the transceiver devices may vary from an intended output transmit power for many reasons. The PSVB/PSMB setting for a base station may be incorrect. The individual transceiver power setting may be incorrect. One or more of the components involved in the transmit path of the transceiver may be malfunctioning, or any combination of these things could occur. Under any of these scenarios, the system may perceive a neighboring base station as being a better server for a particular mobile station when, in fact, its current serving base station is still superior or vice versa.
For example, assume that a neighboring base station is selected as a better server due (at least in part) to the strength at which its control channel is received by a mobile station. Then, when the mobile station is assigned to a traffic channel that is transmitted using a different transceiver, the mobile station actually receives a relatively weak signal because the traffic channel transceiver is transmitting at a much lower power level than expected, e.g., due to a fault in the transceiver, because the PSVB/PSMB setting(s) is incorrect, and/or because the individual power setting of that transceiver has been set incorrectly. If this occurs, then measurements taken subsequent to the handoff will indicate that a different base station is the best server and the mobile will be handed off again, which results in poor call quality and is an inefficient use of resources. Conventionally, these problems associated variances between intended output transmit powers and actual output transmit powers of transceivers have been addressed empirically, i.e., generally by operators manually observing cell performance.
Accordingly, there is a need to develop enhanced techniques to monitor power outputs and power settings of transceivers in radiocommunication systems more accurately, so that processes, such as handoff, can be performed in an efficient way to avoid oscillating connections caused by the xe2x80x9cping-pongxe2x80x9d effect described above.
These, and other, problems, drawbacks, and limitations of conventional output power monitoring techniques, are overcome according to the present invention in which exemplary embodiments provide for a comparison between the signal strength of the measurement channel of a candidate base station and the signal strength of the traffic channel associated with that measurement channel. More specifically, the mobile station can measure the signal strength of the candidate base station prior to handoff and report that signal strength to its current base station. The base station can report the signal strength of the measurement channel to the MSC in the handoff request. Then, when the mobile station is handed off to a traffic channel associated with the new serving base station, i.e., based upon the earlier reported measurement channel""s signal strength, the signal strength of the traffic channel will also be reported by the mobile station and forwarded to the MSC. The system can then create a statistic which tracks the difference between the measured control channel signal strength and the measured traffic channel signal strength, which statistic can then be used to either manually or automatically adjust the output power of a transceiver, a power setting, or both.
Alternatively, the statistic can be generated by measuring a received signal strength of the mobile station""s original traffic channel prior to handoff and a received signal strength of the mobile station""s new traffic channel after a handoff. These signal strengths are compared in the system to identify output power variances.